In Alzheimer's disease (AD) a 4.2 kd polypeptide referred to as the beta protein or A4 is deposited as amyloid fibrils in senile plaques and the walls of cerebral vessels. The gene encoding A4, which is located on chromosome 21, appears to produce two mRNAs by alternative splicing. One mRNA encodes a novel 695 residue amyloid precursor protein (APP) which includes A4 at positions 597638. The other is identical except for a 168 nucleotide insert that would introduce 56 amino acids C-terminal to Arg288. With in situ hybridization using a probe that recognizes both mRNAs, we have established that there is a marked increase in the expression of the APP gene in nucleus basalis neurons of AD patients. Increased levels of amyloidogenic precursor proteins are frequently observed in the systemic amyloidoses, so it is likely that the increased expression of the APP gene that we have observed plays a role in the amyloid deposition that occurs in AD, and increased expression may well be a critical early event in the evolution of AD pathology. Alternatively, increased expression may be an epiphenomenon that occurs in diseased neurons or even a compensatory response that develops in surviving healthy neurons. To clarify these issues, we propose (1) to evaluate each of the APP mRNAs in neurons of several degenerating and non-degenerating populations in AD and in a variety of controls using the in situ hybridization techniques that we have implemented, (2) to model increased APP gene expression by developing a versatile retroviral system capable of expressing high levels of recombinant APP genes in neurons and a wide variety of other cell types both in culture and in vivo, (3) to employ quantitative immunocytochemistry to evaluate the relationship between increased APP gene expression and the levels of APP and A4 that are produced, and (4) to develop methods for evaluating the post-translational processing of APP in an effort to determine if altered post-translational processing plays a role in the amyloid deposition that occurs in AD.